In a Group III nitride compound semiconductor light-emitting device such as a blue light-emitting diode, especially, in a light-emitting device having a substrate disposed on a lower side, a thin translucent electrode is formed on an almost whole surface of a p-type contact layer having relatively large resistance in order to distribute a current into the p-type contact layer evenly to thereby obtain uniform light emission form the whole area of a light-emitting layer. Because wire-bonding cannot be applied directly to such a thin translucent electrode, a pad electrode is formed thereon (see Japanese Unexamined Patent Publication No. JP-A-H09-320984, and so on).
The present inventors have made an examination of further reduction in thickness of the translucent electrode to enhance light transmittance of the translucent electrode. As a result, the following problem to be solved has been found.
That is, as described in the aforementioned patent publications, it is necessary to apply a heat treatment in order to secure ohmic contact between the translucent electrode and the p-type contact layer, secure translucency of the translucent electrode and further secure adhesion of the pad electrode to the translucent electrode and the p-type contact layer. It is conceived that when the heat treatment is carried out, the material of the pad electrode and the material of the translucent electrode are alloyed so that the pad electrode and the translucent electrode are substantially integrated with each other. Hence, a circumferential surface of the pad electrode becomes a surface connected to the translucent electrode.
Incidentally, the p-type contact layer made of a Group III nitride compound semiconductor is different in linear expansion coefficient of formation material from the translucent electrode/pad electrode (p-type electrode structure) integrated with each other by the heat treatment. Because the linear expansion coefficient of the latter made of metal is higher than that of the former, tensile stress occurs in the p-type electrode structure after the heat treatment. For this reason, there is a possibility that the interface between the circumferential surface of the pad electrode and the translucent electrode may crack because this portion is inferior in mechanical strength. Because it is conceived that the current allowed to be applied to the pad electrode (allowable current) is proportional to the area of contact between the pad electrode and the translucent electrode, the allowable current is reduced if there is such a crack. While both greater reduction in chip size and improvement in efficiency of light emission to the outside are required, reduction in size of the pad electrode is also required. On the other hand, maintenance of the status quo of the allowable current or increase of the allowable current is required. It is however impossible to satisfy these requirements if there is such a crack. This is because the area of contact between the pad electrode and the translucent electrode is not sufficient.
Moreover, because the area of contact between the translucent electrode and the circumferential surface of the pad electrode is reduced in the case were reduction in thickness of the translucent electrode advanced, this point is also a factor to cause limitation in the allowable current.
In order to examine the life and durability, a high current may need to be applied to the light-emitting device under a high-temperature environment. In this case, if there is a crack between the circumferential surface of the pad electrode and the translucent electrode, the allowable current is limited as described above. There is therefore a Possibility that originally required stringent examination cannot be executed because the current allowed to be applied at the time of examination is limited. In addition, there may be a disadvantage that burning or the like occurs in the cracking portion at the time of execution of examination so that durability of other members of the light-emitting device cannot be substantially examined any more. This means that long-term durability is poor in the case where a high current is applied under a high-temperature environment such as the outdoors.